Conventionally, various types of active vehicle suspension systems have been proposed. An active vehicle suspension system actively controls a vertical relative movement between a wheel and a vehicle body as the vehicle travels on various kinds of road surfaces with a linear actuating force generated by hydraulic pressure. One such example is disclosed in a Japanese patent laid open publication No. 62-1611.
In the known hydraulic actuator used in the above mentioned active suspension system, a piston is slidably received in an enclosed cylinder so as to define two oil chambers above and below the piston, the piston facing the two oil chambers with corresponding pressure receiving surfaces having different areas, so that a certain thrust may be produced between the cylinder and the piston rod according to the difference in the areas of the pressure receiving surfaces even when the pressures supplied to the two oil chambers are equal to each other. The difference in the areas of the pressure receiving surfaces is typically accounted for by the cross sectional area of the piston rod.
Typically, a bottom of the cylinder is connected with a wheel while an end of the piston rod is connected with a vehicle body, and the output pressure from the pump is directly fed into the upper piston chamber. The pressure of the lower piston chamber is feedback controlled by a pressure control valve comparing it with a certain target value which is determined according to the change in the stroke and the output speed of the hydraulic actuator so that the height of the vehicle body can be kept at a certain constant level with the wheel following the contour of the road surface.
In the above mentioned hydraulic control device, however, when the pump is not operative and the supply pressure to the hydraulic actuator is lost, it is necessary to prevent a drastic change in the height of the vehicle body. To achieve such an end, it has been proposed to disconnect the oil supply passage to the hydraulic actuator from the output of the oil pump with a switch valve and communicate the upper and lower piston chambers with each other so as to place the hydraulic actuator under pressurized condition even when the oil pump is not operative.
According to such a proposal, however, since the two pressure receiving surfaces of the piston are not equal to each other, the downward and upward thrusts acting upon the piston are not equal to each other, and an upward thrust is produced from the hydraulic actuator when the supply pressure is simply trapped in the hydraulic actuator. It is conceivable to reduce the line pressure by using a relief valve when the control action is terminated, but an accumulator for compensating the oil supply capacity is normally connected to the oil pressure supply line, and it is not possible to evenly reduce the pressures in the two piston chambers, thereby creating the tendency that the hydraulic actuator lowers the height of the vehicle when the oil pump is made inoperative.